1. Field of the Invention
The present invention relates to a V-belt continuously variable transmission (CVT) for transmitting a driving force of an engine to a driving wheel, and a straddle-type vehicle having the V-belt CVT disposed below a seat of the vehicle.
2. Description of the Related Art
Recently, there is an increasing demand for straddle-type vehicles called “ATVs (all-terrain vehicles).”
Also, there have been proposed various types of ATVs in which four wheels are provided, each wheel having a wide and low-pressure balloon tire or the like on the left and right sides of the front and rear ends of a body frame, the upper portion of the body frame is provided with steering handlebars, a fuel tank, and a straddle-type seat, arranged sequentially from the front wheel side to the rear wheel side, and a V-belt CVT disposed below the seat for transmitting a driving force of an engine to the driving wheels (see JP-A-2004-156657, for example).
The above-described V-belt CVT includes a primary sheave disposed on a primary sheave shaft, to which a driving force of the engine is input, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; a secondary sheave disposed on a secondary sheave shaft, from which a driving force for the driving wheels is output, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; an endless V-belt received in the respective V-grooves of the primary sheave and the secondary sheave to transmit a rotational driving force between both the sheaves; and a sheave drive mechanism for displacing the movable sheave half of the primary sheave in the axial direction to control the speed change ratio through resulting variations in respective groove widths of the primary sheave and the secondary sheave.
A previous type of sheave drive mechanism, in general, was the so-called centrifugal type which utilized centrifugal force produced according to the engine speed to displace a movable sheave half in the axial direction.
However, when the road on which the vehicle is running turns from a flat road to a climbing road, for example, the sheave drive mechanism of the centrifugal type does not perform a groove width control operation (specifically, a speed change operation) responsively according to changes in the road condition, irrespective of the intention of the rider, until the engine speed has actually decreased because of the increased load from the road. That is, the sheave drive mechanism has a problem of delayed response.
In order to solve the problem of delayed response, another type of V-belt CVT for use in scooter-type motorcycles has been proposed, which includes an electric motor; a control device for controlling the electric motor; a sheave drive mechanism for transmitting a driving force of the electric motor to the movable sheave half of the primary sheave to adjust the respective groove widths of the primary sheave and the secondary sheave; and a rotational speed sensor for detecting rotation of the primary sheave or the secondary sheave to inform the control device of the detected rotation so that the control device can perform a control according to the engine operating condition (see JP-B-2852994, for example).
As the sheave drive mechanism for use in this type of V-belt CVT, there has been proposed the use of a combination device of an electric motor and a ball screw mechanism to move a movable flange in order to control the respective groove widths of the primary sheave and the secondary sheave (see JP-A-2002-227947, for example).
Such a sheave drive mechanism using a ball screw mechanism can achieve a high transmission efficiency and reduce the load on the electric motor.
However, foreign matter entering the ball screw mechanism can damage the surfaces of the ball grooves and balls, deteriorating its durability. That is, foreign matter such as dust can enter a transmission case, in which the sheave drive mechanism of the V-belt CVT is installed, from an external space through air inlet and outlet ports provided in the transmission case for cooling purposes. Also, metal powder or the like produced through wear of the CVT can exist as foreign matter in the transmission case.
Therefore, a dustproof member such as felt is provided to block the gap between the outer peripheral surface of a female ball screw tube and the inner peripheral surface of an enclosing part so as to allow the passage of air therethrough but prevent foreign matter such as metal powder and dust from entering the ball screw mechanism.
With the increasing recent demand for ATVs, there is a need for V-belt CVTs for use in ATVs to electrically control the respective groove widths of the primary sheave and the secondary sheave in order to improve the response during the speed change process.
However, different from scooter-type motorcycles, ATVs are often used off-road and are thus at a high risk of water entering the transmission case through the air inlet and outlet ports for cooling purposes when running over deep puddles, down river sides, etc.
In the case where the sheave drive mechanism using the ball screw mechanism as described above is used in a V-belt CVT for ATVs, for example, the dustproof member may not be able to prevent water from entering the ball screw mechanism and may let water into the ball screw mechanism. Once water enters the ball screw mechanism, grease sealed therein may flow out thereby significantly deteriorating the durability of the sheave drive mechanism.
As an alternative to the above-described dustproof member, a seal member such as an oil seal with high waterproof performance may be provided to block the gap between the outer peripheral surface of the female ball bearing tube and the inner peripheral surface of the enclosing part. In this case, however, axial movement of the female ball screw tube causes expansion and contraction of the space sealed by the seal member and hence changes in pressure therein.
Thus, the oil seal cannot exhibit sufficient seal performance with its lip curled. In addition, the female ball screw tube may bear an increased load, and as a result, the movable sheave half cannot be fed smoothly and the load on the electric motor will be increased.